Shift register



Nov. 18, 1958 w. A. ENGLAND SHIFT REGISTER Filed Aug. 27, 1956 FIGQI STAGE 1* 3 STAGE =H= 2 STAGE #l INVENTdR WILLIAM A. ENGLAND BY W AGFNT SHIFT REGISTER William A. England, Morris Plains, N. 1., assignor to Monroe Calculating Machine Company, Orange, N 3., a corporation of Delaware Application August 27, 1956, Serial No. 606,397

' 2 Claims. c1. sis-84.5

This invention relates to a shift register which uses a single current controlling element per stage. The current controlling element may be a cold or heated cathode type gas filled tube, a vacuum tube, or a transistor.

The shift register of this invention is of the type which is used in digital computers and other digital data handling equipments to store or delay items of information or to convert binary coded information from serial to parallel form or from parallel to serial form. Shift registers of this type are disclosed in Patent No. 2,601,089 to William H. Burkhart and'Patent No. 2,638,542 to Howard M. Fleming, Jr. .When using, either cold or heated cathode types of gas filled tubes the shift register of this invention provides a sufiicient power output to operate relays or other devices directly from each stage without additional amplification. The cold cathode type tube provides a very inexpensive shift register for relatively low speed operation. The heated cathode or thyratron type tube provides a faster operation but is more expensive than the cold cathode type. register can handle information. very rapidly but does not have an unlimited storage time.

An object of the invention is to provide a shift register of the type described which is less expensive than others presently known;

-A further object of the invention is to provide a shift register of the type described which can'provide a power output from each stage that is capableofoperatingrelays or other mechanisms without additional amplification. Other objects and a fuller understanding of the invention may be had by referring to the following description and claims, taken in conjunction with the accompanying drawings in which: V

Fig. 1 is a schematic circuit diagram of three stages of a shift register using cold cathode tubes and forming an embodiment of the invention.

Fig. 2 shows a modification to the circuit of Fig. 1 using vacuum tubes.

Fig. 3 shows a junction transistor connected in place of a vacuum tube in the circuit of Fig. 2.

.Fig. 4 is a schematic circuit diagram of another embodiment of the invention using thyratron type tubes. 7

Fig. 5 shows a modification to the circuit of Fig. 4 usin vacuum tubes. I p s Referring now to Fig. 1, each stage of the shift register has a cold cathode type gas filled tube 11 with plate, cathode and starter electrodes. The operation of tubes of this type are described in the Bell Telephone System Technical Publication, Monograph B1685, Circuits for Cold Cathode Glow Tubes, by W. A. Depp and W. H. T. Holden. The supply voltage on line 12 will not start conduction between plate and cathode of tube 11 but will maintain it once it is started. Conduction of a tube 11 can be started by raising the voltage on the starter When using vacuum tubes or transistors, the shift electrode above a breakdown value that causes the gas to r 2,861,216 Patented Nov. 18,1958

plate voltage below a given value long enough to allow the gas to de-ionize.

The information to be handled is in serial binary code form wherein each binary digit occupies a given time interval. Each 1 digit is represented by a positive pulse occurring during its digit interval and each 0 digit by the absence of a pulse. The 1 digit pulses may be short in time duration compared to the digit interval. The information is applied to terminal 13 which is normally held below the starter breakdown potential. Each positive pulse raises the starter electrode of tube 11 in the first stage above the breakdown potential and causes it to conduct between plate and cathode. The tube 11 will then continue to conduct until it is extinguished by re-' moval of its plate voltage.

Negative and positive going clock pulsesioccur con: currently at the end of each digit interval and are short in time relative to the digit intervals. The negative going clock pulses are applied to line 12 and extinguish all conducting tubes 11. The positive going pulses are applied to line 14 and each sets the state of conduction of each stage, except the first, to the reverse of the state'of conduction of the preceding stage during the previous digit interval. The manner in which this is accomplished will be described later.

Tube 11 in the first stage will be caused to conduct by each 1 digit pulse applied to line 13 as previously explained. Current through its plate load resistor 15 will lower the voltage on its plate. Condenser 16 connected to its plate through resistor 1-7 will then discharge to the lowered plate voltage. The next negative going clock pulse applied to line 12 will then further lower the plate voltage and extinguish tube 11. If the next digit is a 0, tube 11 willremain non-conducting during the digit interval and its condenser 16 will charge to the supply potential through resistors 15 and 17; The time constant of this circuit is long enough to prevent condenser 16 from discharging appreciably during a negative clock pulse interval. Y The state of charge on condenser 16 in the first stage at the end ofeach digit interval thus indicates whether the digit was a l or a 0. This state of charge will determine the potential to which condenser 18 in the second stage will be charged by the positive clock pulse. The positive clock pulse raises line 14 from ground to a positive potential and charges each condenser 18through a rectifier 19 and a resistor 20 in parallel with a resistor 24. As soon as the charge on the second stage condenser 18 rises to that on first stage condenser 16,- the condenser 16 will also charge through rectifier 22. As condenser 16 is considerably larger than condenser 18, the charging time constant through rectifier 19 and resistor 20 will be considerably increased. The further increase in charge on condenser 18 Will then be small.

If the first stage condenser 16 was discharged; the charge on the second stage condenser 18-will not reach the starter breakdown potential before the positive clock pulse ends. If the condenser 16 was charged substantially to'the supply voltage level, however, the charge on the condenser 18 will rise above the starter breakdown potential. The second stage tube 11 will then ionize and conduct between plate and'cathode when the negative clock pulse ends and plate voltage is againapplied. Resistor 24 allows the charge on condenser 18 to leak off after tube 11 starts to conduct. Resistor 23 limits the starter current to prevent damage'to'tube 11.

The state of conduction of tube 11 in the second stage during a digit interval will thus be the reverse of the state of conduction of tube 11 in the first stage during the register, the serial information entered on terminal 13 is advanced one stage at the end of each digit interval. The odd numbered stages register 1 digits by conducting and digits :by not conducting. The even numbered stages register 0 digits by conducting and 1 digits by not conducting.

Information passing through the shift register will be elayed by as many digit intervals as there are stages. A much greater delayv of a group of digits can be provided by stopping the application of the clock pulses. The stages of the shift register will then maintain their conducting state as long as power is applied and the information registered can be stored indefinitely.

To convert information from serial to parallel forr'n, outputs can be taken individually .from each stage after the serial information is read in. This can he done, for example, by'conn'ecting one side of a set of relays or other actuating devices individually to the plates of tubes 11. A gating arrangement would then initiate parallel readout by connecting the other sides of the odd numbered devices to the supply voltage and of the even numbered devices to the same potential which is reached by the plates of tubes 11 when they are conducting.

As the odd numbered tubes v11 register 1 digits by conducting and the even numbered by not conducting, the relays or other actuating devices would then be energized only when their respective stages registered 1 digits. If relays are used to receive the parallel outputs it would also be possible to arrange their contacts to compensate for the different way in which the odd and even numbered stages register the binary digits. The odd numbered relays could have make contacts where the even numbered relays have break contacts and vice versa. The other side ofall relays could then be connected to the same potential by the gating arrangement and the contacts of both odd and even numbered relays would indicate digitsin the same manner.

To convert information from parallel to serial form, the digits would have to be applied to the inputs of the stages individually. In this shift register, however, it is necessary to reverse the even numbered digits so that a pulse would be received if the digit were a 0 and not if a 1. Before information can be entered in parallel, it is also necessary to place all tubes 11 in a non-conducting state. This can be done by applying a negative clock pulse and suppressing the accompanying positive clock pulse.

Heated cathodetype gas filled tubes, such as thyratrons, can be substituted for the cold cathode type tubes 11 in the circuit of Fig. 1 by adding means to bias the grids negatively. This can be done, for example, by connecting the junction of each starter electrode, or grid, and resistor 23 to a negative potential through a decoupling resistor. Resistor 23 and the decoupling resistor would 'then form a voltage divider between the charge on condenser 18 and the negative bias potential and the tubes would be held cut ofl unless condenser 18 were charged to a high positive value. Operation would be the same as with cold cathode type tubes 11. Information could be handled at a faster rate as less timewould be required for ionization and de-ionization.

A shift register capable of much faster operation is provided by replacing gas filled tubes 11 with vacuum tubes 21 and making the other modifications shown in Fig. 2. Resistors 2t) and 24 are eliminated and'the lowersides of condensers 18 are connected to line 25 instead of to ground. The negative going clock pulses applied to line 12 in the circuit of Fig. 1 are replaced by a second set of positive going clock pulses applied to line 25. Each pulse of this second set starts just as each pulse of the first set applied to line Blends. Line 14 is biased negatively while line 25 is held at ground potential during the absence of a pulse.

At the end of each'di'git interval, the positive clock pulse applied to line 14 charges all'condensers i8 through rectifiers 19 to a positive potential. The positive clock pulse of the second set then raises the other side of condensers 18 positively. The grids of all tubes 21 Wlll be held positive during both clock pulses and the tubes 21 will conduct. The duration of both clock pulses will be short, however, so that the conduction of tubes 21 during the application of the clock pulses will not change the charge on condensers 16 appreciably. The digit intervals must be longer than the two clock pulses so that the charge on each condenser 16 at the end of the digit interval will reflect the state of conduction of its associated tube 21 during the digit interval. Resistors 23 will be large so that grid current will prevent the grids from being driven far enough positive to damage tubes 21.

As in the circuit of Fig. l, a condenser 16 will be charged to a high potential if its associated tube 21 was cut 011 during the preceding digit interval and to a low potential it its associated tube 21 was conducting. Condensers 16 are considerably larger than condensers 18 and rectifiers 22 will conduct to prevent the potential on the upper side of a condenser 18 from rising much above the potential'on the condenser 16 in the preceding stage. if a condenser 16 is at a low potential, the condenser 18 in the following stage will discharge through rectifier 22 as the positive clock pulse applied to line 25 raises its lower side positive. The condenser 18 will then have a negative charge when the clock pulse ends. If a condenser 16 is charged to a high potential, condenser 18 in the following stage cannot discharge through rectifier 22 and will retain its positve charge.

When the clock pulse on line-25 ends, each condenser 18 will hold the grid of its tube positive or negative depending upon whether the condenser 16 in the preceding stage was charged to a high or low potential. If the time constant of condenser 16 and resistor 17 is long relative to the clock pulse duration, the difference in potential on a condenser 18 for the two conditions can be nearly equal to the difference in plate potential when tube 24 is conducting and when it is cut 0E. A positive charge on condenser 18 will keep its tube '21 conducting while a negative charge will cut it off. The state of conduction of each tube 21 during a digit interval is thus the reverse of the stage of conduction of the tube 21 in the preceding stage during the previous digit interval just as in the circuit of Fig. l.

Resistors 23 will be in the order of megohms so that the flow of grid current will limit the voltage on the grids to substantially ground potential when condensers 18 are further positive. There will then be no substantial reduction in conduction of a tube until the charge on its condenser 18 -leaks oif below ground potential. When charged negatively, a condenser '18 can hold the grid of its tube 24 well below cut-01f so that again no change of conduction will take place until there has been aconsiderable leakage of charge from condenser 18. While information cannot be stored indefinitely, as is possible with gas filled tubes, the charge on condensers 18 will hold for the, short time storage required in many applications.

Vacuum tubes 21 in the circuit of Fig. 2 can be replaced with NPN junction type transistors '31 as shown in Fig. 3. Component sizes and voltages willchange but operation will otherwise be similar. Unlike the grid of a vacuum tube 21, however, the base of a transistor 31 must draw current to cause conduction between the other electrodes. This will more quickly dissipate the charge on condensers 18 and considerably reduce the time during which information can be stored. PNP junction transistors could be used in .place of the .NPN types by reversing the connections to rectifiers .19 and 22 and changing the polarity of supply voltages and clock pulses.

Each stage of the circuit of Fig. 1 can also be modified as shown in Fig. 4 to ,provide ;a shift register which registers digits the same way on each stage instead of inverting them on alternate stages. Inverting means for alternate stages are thus not required for reading information in or out in parallel. A heated cathode type gas filled tube 41, such as a thyratron, is shownin each stage. As will be explained later, however, a cold cathode gas filled type could be used as well. The modification consists mainly of removing rectifiers 22 and adding condensers 26. Each condenser 26 is connected between a condenser 16 and the junction of rectifier 19 and resistor 20 in the next stage. Line 14 is biased negatively and positive clock pulses are not used.

In the circuit of Fig.4, as in the circuit of Fig. 1, the negative clock pulses applied to line 12 cut 011 all tubes 41 at the end of each digit interval. The "charging time constants for condensers 16 through their associated resistors 15 and 17 are long with respect to the clock pulse duration. If a tube 41 was cut 011 during the previous digit interval, the voltage across its associated condenser 16 will change only slightly when the negative clock pulse ends. If a tube 41 was conducting, its associated condenser 16 will be discharged and the voltage across it will rise as it charges when the negative clock pulse ends.

The rising voltage across a condenser 16 is used to trigger the next stage into conduction. As the condenser 16 charges, the condenser 26 connected to it will also charge. The resistors 20 and 24 are relatively large so that most of the charging current for condenser 26 is drawn from the associated condenser 18 through a rectifier 19. The rising voltage across condenser 16 will thus be substantially distributed across the associated condensers 26 and 18 in inverse proportion to their size. The positive charge on the condenser 18 will cause its associated tube 41 to ignite.

Each tube 41 will thus conduct during a digit interval only if the tube 41 in the preceding stage was conducting during the previous digit interval. As a tube 41 starts to conduct, the voltage across its condenser 16 drops again and charging of the condenser 18 in the next stage is stopped. Due to variations in tube characteristics and circuit components, this may occur before tube 41 in the next stage ignites ifit'also was previously conducting. The voltage on the plate of this tube 41 will continue to rise as its condenser 16 charges so that it will be ignited by the charge on its condenser 18 at a slightly later time.

If it is desired that all tube 41 which are to conduct ignite at more nearly the same time, a delay can be added between condensers 18 and the grids of tubes 41. The delay would permit condensers 18 to be charged far more positively than necessary and the plate voltages to rise substantially before any tube 41 ignites. Variations in the time of ignition would then depend almost entirely on the slight variations in the delay circuits. Such delays could be provided, for example, by adding condensers between the grid of tubes 41 and ground.

The charge on condensers 18 leaks oif through resistors 24 during each digit interval so that a tube 41 will not be caused to ignite unless its condenser 18 has just received a charge. Condensers 26 discharge through resistors 20 during the digit interval so that most of an increase in voltage across a condenser 16 is always applied to the condenser 18 in the following stage. The discharging time constants for condensers 26 could be reduced by replacing resistors 28 with rectifiers connected to offer a high impedance for charging and a low impedance for discharging.

Line 14 is biased negatively to hold tubes 41 cut off unless their condensers 18 are charged positively as previously explained. If cold cathode type gas filled tubes were to be used in place of heated cathode type tube 41, line 14 would be biased positively just below the potential necessary to maintain ionization between starter and cathode. This would reduce the charging which would be required on a condenser 18 to raise the starter electrode above the breakdown potential.

The circuit of Fig. 4 can also be modified as shown in Fig. 5 to operate with a vacuum tube 51 instead of a gas filled tube 41. Condensers 16 and resistors 17 and 24 are removed. Rectifiers 27 are added and connected between taps on resistors 23 and a line 28. Condensers 26 are connected directly to the plate of the tube 51 in the preceding stage. The negative going clock pulses are applied to line 28 instead of to line 12. The effect of each clock pulse is the same, however, in that it cuts' oif all tubes 51. v 7

If a tube 51 is conducting, the voltage on its plate will rise when it is cut off by a negative clock pulse applied to a tap on its grid limiting resistor 23 through a rectifier 27. The rising voltage will charge condensers 26 and 18 in the following stage through their associated rectifier 19. The resulting positive charge on condenser 18 will cause its associated tube 51 to conduct as soon as the negative clock pulse ends. As in the circuit of Fig. 2, resistor 23 is large so that the grid of tube 51 remains substantially at ground potential and there is little change in conduction until the charge on condenser 18 falls below ground potential.

If a stage was not previously conducting, the clock pulse will produce no change and there will be no rise in voltage on its plate. Condenser 18 in the next stage will then be charged negatively through a section of resistor 23 and its associated rectifier 27 by the negative going clock pulse. This negative charge on condenser 18 will then continue to hold its tube 51 cut off after the negative clock pulse ends.

Like the circuit of Fig. 2, the circuit of Fig. 5 cannot store information in a stationary manner indefinitely as the imposed charges on condensers 18 will eventually leak off. The circuit of Fig. 5 will also work with transistors 31 connected as shown in Fig. 3 in place of tubes 51. As will be understood from the description of the circuit of Fig. 5, the circuit of Fig. 4 would also operate if condensers 26 were connected directly to the plates of tubes 41 and condensers 16 and resistors 17 were eliminated.

Although the invention has been described with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example and that numerous changes in the details of construction and the combination and arrangements of parts may be resorted to without departing from the spirit and scope of the invention as hereinafter claimed.

What is claimed is:

1. A shift register circuit comprising a series of gaseous discharge devices each having a cathode, a main anode and a starter anode, said cathodes being connected to afirst potential, said main anodes being. connected to a second potential sufficiently positive relative to said first potential to sustain a discharge in said devices, means for rendering at least the first device in said series conductive, a first plurality of condensers each connected between the main anode and the cathode of a respective device, a second plurality of condensers each connected between the starter anode and the cathode of a respective device, means for momentarily decreasing the potential applied to said main anodes and for charging each second condenser in accordance with the charge on the first condenser of the next preceding device.

2. A shift register circuit comprising a series of gaseous discharge devices each having a cathode, a main anode and a starter anode, said cathodes being connected to a first potential, said main anodes being connected to a second potential sulficiently positive relative to said first potential to sustain a discharge in said devices, means for rendering at least the first device in said series conductive, a first plurality of condensers each connected between the main anode and the cathode of a respective device, a second plurality of condensers each connected between the starter anode and the cathode of a respective device, a plurality of unilateral conducting devices each connected between a respective main anode connected terminal of 7 2,861,216 7 '8 one ofsaid first condensers and the starter anode con- References Cited in the file of "this patent nected terminal of the one of said second condensers of UNITED STATES PATENIS the next succeeding discharge device, said unilateral conducting devices being poled for current flow toward'sai'd 2,760,087 Felkar A 2 1955 first condensers, means for momentarily decreasing the 5 2,790,109 Ruhllg P 1957 potential applied to said main anodes and for applying 7 p a potential at least aspositive as the starter breakdown OTHER REFERENCES potential to said starter anode connected terminal of each P li i n l T e Transistor g ti of said second condensers for charging each second con- Amplifier as a Computer Element by G. B. B. Chaplin,

denser in accordance with the charge on the first con- 10 The Proceedings of the I. E. B, vol. 101, part III, No.73, denser of the next preceding discharge device. pp. 298-397, only p. 304 necessary, September 1954. 

